Horn antennas have been used to receive RF signals for decades and many different types of horn antennas have been designed and used. A simple horn antenna has a rectangular cross-section which tapers in cross-section from the front of the antenna to the back of the antenna, causing the dominant mode of the received electromagnetic energy to smoothly transition from incoming plane-wave-like energy for receipt. The electromagnetic energy travels from the front of the antenna to the back of the antenna without any discontinuity or reflection so that the dominant mode of the electromagnetic energy is transformed to substantially match a waveguide, a microstrip, a stripline or some other convenient form of transmission line that carries the RF signals to a receiver.
A major disadvantage of simple horn antennas is that when they are used at frequencies whose wavelengths are less than either the major or minor dimensions of the cross-sections of the antennas, higher order modes become possible. This is true whether an antenna has a rectangular cross-section, an elliptical cross-section or some other operable cross-section. The higher order modes have pattern maxima that are at an angle to the centerline of the antenna. As a result of the higher modes, the antenna gain, which is a measure of the energy proceeding along the centerline as a fraction of the total energy, is reduced.
To prevent the higher order modes, know as “over-moding,” simple horn antennas are restricted to reception of frequency ranges of less than one octave. To overcome the restricted frequency range, ridged waveguide horn antennas have been developed. FIGS. 1A, 1B and 1C show cross-sections of a simple rectangular waveguide 100, a single-ridge waveguide 102, and a double-ridge waveguide 104, respectively.
The addition of one ridge 106 or two ridges 108 changes the relationship between the frequencies of the modes. In the rectangular waveguide 100 or antenna of FIG. 1A, the fundamental mode must have a frequency fc of at least:
                              f          c                =                  c                      2            ⁢                                                  ⁢            a                                              Equation        ⁢                                  ⁢        1            where a=the long side length and c=the speed of light in the medium, and the mode in question is vertically polarized (the small side 110 being defined as the vertical side in FIG. 1A). Higher order modes are possible when the frequency is more than twice this minimum frequency fc. The impedance of the antenna gradually increases as the frequency of operation is reduced to fc until it approaches infinity at fc, so that the practical frequency range is more likely to be about 1.3fc to just under 2fc.
The value of fc determines the minimum size to which the horn may taper at the back of a simple horn antenna. This size is given by the inverse of Equation 1 above and is shown in Equation 2:
                              a          min                >                  c                      2            ⁢                                                  ⁢                          f              c                                                          Equation        ⁢                                  ⁢        2            For practical purposes, the minimum size is usually taken to be about 30% greater than that given by Equation 2 in order to keep the impedance from becoming too high to match to the following circuit. In order to increase the range of operational frequencies of a horn antenna, one or two ridges can be formed in the horn antenna as shown in the cross-sectional views shown in FIGS. 1B and 1C, respectively. When ridges are used, the relationship between the higher mode frequencies and the fundamental mode frequency is no longer a simple multiple. With a suitable choice of ridge width and gap (measured from the top of the ridge 106 to the antenna “ceiling” 112 in FIG. 1B, or between the two ridges 108 in the FIG. 1C), a frequency range of more than one octave can be achieved without over-moding, and some ridged waveguides can pass frequency ranges of 3fc, 4fc or even greater.
The challenge of transferring electromagnetic energy from the waveguide or antenna to another medium must be addressed when using ridged horn antennas. More particularly, with the advent of solid state devices that are usually implemented on microstrip printed circuit boards and stripline printed circuit boards, a method of effectively transferring received RF signals from ridged horn antennas to an associated printed circuit board is needed. Novel methods and apparatus for these transfers is the subject of the present application.